When a function such as an attenuation function, a polarization function, a scattering function and a light-shielding function is required to effect for an irradiated light including a laser light and a natural light employed, hitherto, an apparatus or device which operates based on a different principle was adapted depending on the function required. Accordingly, products corresponding to the functions were prepared respectively by production processes that were different depending on the respective functions.
For example, in LCD (liquid crystal display), linear polarizing plates or circular polarizing plates are used to control optical rotation or birefringence in display. Also in OLED (organic electroluminescence device), circular polarizing plates are used to prevent reflection of external outside light. Heretofore, for such polarizing plates (polarizing elements), iodine has been widely used as a dichroic substance. However, if iodine is used for a polarizing plate, its heat resistance or light fastness is insufficient since iodine is highly sublimable. Further, the extinction color becomes dark grayish blue, and an ideal achromatic color polarizing plate for the entire visible spectral region cannot necessarily be obtained.
Therefore, a polarizing element has been studied in which an organic dye is used as a dichroic substance which replaces iodine. However, such an organic dye has a problem such that only polarizing elements are obtainable which are distinctly inferior to those employing iodine for dichroic property. Particularly, a polarizing element is an important constituent in LCD employing, as the display principle, optical rotation or birefringence of light, and a new polarizing element has been developed for the purpose of improving display performance and the like in recent years.
As one method of forming such a polarizing element, a method is proposed in which, in the same manner as in the case of a polarizing film containing iodine, an organic dye having dichroism (dichroic dye) is dissolved or adsorbed in a polymer material such as a polyvinyl alcohol, and the thus-obtained film is stretched in one direction into a film so that the dichroic dye is oriented or aligned. However, this method had such a problem that a long time period and efforts are required for e.g. the stretching step.
Thus, other methods attract attention in recent years, and as such methods, Dreyer, J. F., Journal de Physique, 1969, 4, 114, “Light Polarization From Films of Lyotropic Nematic Liquid Crystals” discloses a method of aligning a dichroic dye on a substrate such as glass or a transparent film utilizing e.g. intermolecular interaction of organic dye molecules, to form a polarizing film (anisotropic dye film). However, it was known that there was a problem for heat resistance of the film obtained by the method by Dreyer.
Further, the alignment of a dichroic dye on a substrate such as glass or a transparent film utilizing e.g. intermolecular interaction of organic dye molecules, is attained by a wet system film-forming method. In the case where an anisotropic dye film is prepared by the wet system film-forming method, the dye molecules to be used for the dye film are required not only to show high degree of dichroism but also to be a dye suitable for the process for the wet system film-forming method. Examples of the process in the wet film-forming method include a process of causing sedimentation and alignment of the dye on a substrate, or a process of controlling the alignment. Thus, there are many cases that even the conventional dyes that can be employed for the polarizing elements passing through the above-mentioned stretching treatment are not suitable for the wet film-forming method. On the other hand, JP-A-2002-180052 (“JP-A” means unexamined published Japanese patent application), JP-A-2002-528758 and JP-A-2002-338838 propose materials suitable for the process of the wet system film-forming method. However, although such materials are suitable for the process, they have had such a drawback that they cannot show high dichroism.
Further, JP-T-8-511109 (“JP-T” means published searched patent publication) proposes a dye represented by chromogen (SO3M)n as a material suitable for the process. In JP-T-8-511109, the achromatic color is given by combining several kinds of dichroic dyes each other. However, when an anisotropic dye film is obtained by combining the several kinds of dichroic dyes each other, a molecular alignment for mixing different molecules is disturbed and there was a problem that achieving a high dichroism is difficult.
Also, JP-A-56-64301 describes that a dichroic dye having a vinyl group is used as a dichroic dye, to produce a polarizing film high in fastness. However, a process such as stretching treatment is required in the method described in JP-A-56-64301, similar to the above.